Ceramic materials made from SiAlON have recently been investigated for high temperature industrial applications due to their excellent properties. High toughness, and elevated temperature strength, along with oxidation resistance have made SiAlON ceramics the perfect candidate for certain applications. Prior art compositions and methods have attempted to rid the SiAlON materials of glass phases incorporated into the material as they have been the typical cause of high temperature degradation. Attempts have been made to remove oxide sintering additives because they form glasses which remain in grain boundaries, and reduce the overall strength of the material.
In an attempt to provide a ceramic SiAlON composition which is usable in high temperature applications, prior art methods and compositions have taught the combination of alpha-SiAlON with beta-SiAlON because the beta-SiAlON contained elongated lath-like microstructures. The beta-SiAlON material is elongated, and therefore adds strength to the material. However, certain residual glass phases were formed in the grain boundaries between the alpha and beta phases when the prior art methods were performed. These residual glasses are undesirable as they have been the typical cause of high temperature degradation as discussed above. One can see that it would be advantageous to provide a multi-phase SiAlON material without the residual glasses.
In addition, strontium was not generally incorporated into the alpha-SiAlON structure in the prior art since it was thought that cationic strontium atoms were too large to fit interstitially into position within the alpha-phase matrix. None of the prior art found in the relevant patent search teaches a multi-phase SiAlON material with an alpha phase which contains strontium and also has elongated lath-like grains.
Various multi-phase SiAlON sintered bodies containing both alpha- and beta-SiAlON are known in the art and are described in the following patents. However, these patents do not teach strontium in the alpha phase.
U.S. Pat. No. 4,880,755 issued to Mehrotra in 1989, discloses a SiAlON ceramic material comprising a ceramic core selected from alpha-SiAlON, beta-SiAlON, and an intergranular phase and, optionally, substantially inert refractory phases. The SiAlON ceramic material has an alloyed surface layer with a substantially increased aluminum and oxygen content.
European Patent Application No. 84302052.0 published Nov. 14, 1984 teaches a mixed phase sintered SiAlON prepared with additions of yttria, alumina and aluminum nitride useful for engine parts. The constituent phases were represented by alpha-SiAlON and beta-SiAlON, wherein the ratio between both phases is set at 0.05-0.7:0.95-0.3, or by an alternative composition comprising alpha-SiAlON, beta-SiAlON and (Si-RE-Al-ON) where RE is a rare earth metal and wherein the ratio of the alpha-SiAlON phase to the whole of all constituent phases is set at 0.01 to 0.7.
In addition to the above-referenced patents which teach SiAlON materials having combinations of alpha and beta-phases, there are a number of patents which teach the use of strontium oxide as a sintering aid and which teach silicon nitride bodies containing strontium. These compositions and methods are set forth in the following patents.
U.S. Pat. No. 4,870,036 issued to Yeh in 1989 teaches an improved silicon nitride material characterized by high mechanical strength at high temperatures comprising a combination of approximately 0.5-6.0 percent by weight of strontium oxide, 2.0 to 12.0 percent by weight of yttrium oxide and the balance silicon nitride. The material is densified by either an encapsulated hot isostatic pressing method or an encapsulated sinter/HIP method and can be utilized to form near net shape articles.
U.S. Pat. No. 4,692,420 issued to Oda in 1987 teaches a silicon nitride sintered body consisting essentially of Mg, Sr, Ce and Al. These compounds are utilized in the form of their respective oxides in combination with the silicon nitride. The silicon nitride sintered body has a low thermal conductivity, while high mechanical strength and high thermal shock resistance are maintained.
Consequently, it would be a great advantage for a material made of SiAlON which would incorporate multiple phases of both alpha-phase and beta-phase SiAlON materials which could also include elongated, lath-like microstructures in both the alpha- and beta-phases. Although we have seen SiAlON materials having both alpha- and beta-phases, as well as seeing silicon nitride bodies containing strontium, it would be advantageous to provide a SiAlON material which includes multiple phases in which the phases also contain strontium.
Therefore, it is a primary object of the present invention to provide a multi-phase SiAlON ceramic material having at least an alpha-SiAlON phase, a beta-SiAlON phase and an amorphous intergranular phase wherein the alpha-SiAlON phase contains strontium and elongated grains in the ceramic material to enhance the strength.
It is another object of the present invention to provide a multi-phase SiAlON ceramic material which exhibits excellent strength and toughness at high temperatures.